US2988437A - Cyanamide catalyst for ammonium nitrate gas generating composition - Google Patents

Description

2,988,437 CYANAMEE CATALYST FOR AMIVIONIUM NI- TRATE GAS GENERATING COMPOSITION William G. Stanley, Hammond, Ind., and Wayne A.

ProelLjQhicago, BL, assignors to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Filed Oct. 28, 1952, Ser. No. 317,357 7 Claims. (Cl. 52-15) This invention relates to new compositions and also to compositions for the generation of gas at high pressure. Particularly, the invention relates to explosive compositions wherein ammonium nitrate is the principal or sole gas-producing agent. Still more particularly, the invention relates to an explosive comprising ammonium nitrate, an oxidizable material and a cyanamide combustion catalyst.

Ammonium nitrate is widely used as a component of high explosives. Although ammonium nitrate is classified as a high explosive, it is extremely insensitive and cannot readily be detonated by the local application of heat or by a blasting cap; and when ignited, ammonium nitrate does not sustain propagation consistently. Normally ammonium nitrate is mixed with an oxidizable material, such as, sulfur, carbon, cellulosic materials, hydrocarbons, etc., in order to utilize the excess oxygen available in the ammonium nitrate. However, these mixtures of ammonium nitrate and oxidizable materials are also either very insensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity is the use of a sensitive high explosive to prime the detonation of the ammonium nitrate explosive. Examples of suitable primers are tetryl, TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive that is detonable by the action of a blasting cap can be obtained by mixing the ammonium nitrate with the sensitive materials, such as, nitrostarch and nitrocellulose. The extreme sensitivity of these explosives makes them undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrate explosive is to admix therewith an organic sensitizer, such as, nitrogen compounds and certain carbohydrates. In general, satisfactory explosives are obtained only when the sensitizer is very intimately dispersed throughout the mass. Generally this dispersion is obtainable only by the use of complicated and expensive procedures.

The most commonly used method for improving the sensitivity of ammonium nitrate explosives is the addition of a combustion catalyst. The commercially used combustion catalysts are all based on the element chromium. The more common chromium combustion catalysts are ammonium or alkali metal chromates or polychromates; chromic oxide, chromic nitrate and copper chromite. The preferred material is ammonium dichromate. While the chromium compounds are the best known combustion catalysts, they have the disadvantages of being expensive and of, frequently, being in very short supply. The relative toxicity of the chromates makes them hazardous unless handled with considerable care. A particular disadvantage to the use of chromium combustion catalysts is their strong oxidizing. properties, such that they tend to react with the binder upon long storage to give chromium salts which are relatively ineffective catalytically.

An object of this invention is the preparation of a new and improved inorganic nitrate explosive, in particular, an ammonium nitrate explosive. Another object is the preparation of an explosive comprising ammonium nitrate and a cyanamide combustion catalyst. A further object of this invention is a gas generating composition for the generation of gases at pressures on the order of ate 2,988,437 Patented June 13, 1961 1000 p.s.i., which composition comprises ammonium nitrate, an oxidizable material and a cyanamide combustion catalyst.

The above objects and other objects which will be apparent in the detailed description of the invention are achieved as follows: The gas generating composition of this invention comprises a mixture of ammonium nitrate, or a mixture of ammonium nitrate and an oxidizable material, and an effective amount of a combustion catalyst consisting essentially of at least one metallic cyanamide selected from the group consisting of barium, copper, lead, mercury and silver. in addition to the ammonium nitrate, moderate amounts of other inorganic nitrates, such as, potassium nitrate, sodium nitrate and magnesium nitrate may be present. It is preferred IO have present in the high pressure gas generating composition of this invention an oxidizable material, which material utilizes a part or all of the excess oxygen available from the decomposition of the ammonium nitrate and the combustion catalyst. (The catalyst uses some of the excess oxygen from the decomposition of the ammonium nitrate for the oxidation of the metal and carbon content thereof.)

The combustion catalysts of this invention are members of the group of metallic cyanamides that are stable in storage and, particularly, stable in the presence of ammonium nitrate. amide and calcium cyanamide are unsatisfactory for the purposes of this invention because of their lack of stability at atmospheric conditions of temperature. -The stable metallic cyanamides are barium cyanamide, copper cyanamide, lead cyanamide, mercury cyanamide and silver cyanamide.

Barium cyanamide is difficult to prepare in the essentially pure state. The presence of even minor amounts of barium cyanide is undesirable'because of the consequent instability of the barium cyanide in the presence of ammonium nitrate.

The various non-reactive metallic cyanamides are not of equal effectiveness as combustion catalysts for ammonium nitrate. Furthermore, a difference in storage stability exists in the absence of, as well as in the presence of, ammonium nitrate. The preferred combustion catalysts are the metallic cyanamides of copper and lead.

When operating with ammonium nitrate in the absence of any appreciable amount of oxidizable material, large amounts of catalyst are needed to permit smooth burning of the composition. For ignition at ordinary temperatures, the amount of catalyst needed may be as much as 10 Weight percent of the total explosive mixture. However, the ease of ignitability of the mixture increases with increase of pressure in the gas generating chamber and at elevated pressures, ignition can take place with the use of smaller amounts of combustion catalyst. The term ordinary pressure is intended to mean pressure such as exists normally in the atmosphere or in mining operations.

When the gas generating composition consists essentially of ammonium nitrate, an oxidizable material and the combustion catalyst of this invention, the combustion catalyst may be present in an amount between about 1 and 25 weight percent, based on the total composition. In order to improve the ignitability of the composition and obtain smoother burning thereof, it is preferred to use at least about 2% of catalyst. The burning rate is somewhat alfected by the amount of catalyst present in the composition; however, the maximum eifective concentration of the catalyst is about 15%. The amount of catalyst needed to obtain good ignitability and smooth operation is dependent somewhat on the amount and type of oxidizable material present; in gen eral between about 2 and 4 weight percent will give eX-' Sodium cyanamide, potassium cyan cellent results when maximum burning rate is not a main consideration. When maximum burning rate is a main consideration, the amount of catalyst used should be between about 6 and 15 weight percent.

The decomposition of ammonium nitrate produces free-oxygen in addition to the other decomposition products. Additional energy can be obtained by having present in the combustion zone an oxidizable material which combines with the free-oxygen. It is to be understood that the combustion catalysts utilized in the explosive grain of this invention contain oxidizable materials which will consume some of the free-oxygen. The oxidizable material may be any material which contains a deficiency of combined oxygen. Metals such as aluminum and magnesium may be used. The non-metallic elements, sulfur and carbon, may be used. Nitrogen-containing organic compounds that do not unduly sensitize the explosive mixture are particularly good; examples of these are urea, nitroguanidine, mononitronaphthalene, dinitrodiphenyl oxide, etc. Hydrocarbon materials are an excellent oxidizable material and are preferred where low cost is a matter of concern. Examples of hydrocarbon oxidizable materials are waxes, tars, asphalts, bitumen, coal tar, shale oil residue, viscous higher boiling hydrocarbon oils, etc.

Oxygenated materials are very useful as oxidizable materials for certain applications of the explosive grain. For use in rockets, a material that is dimensionally stable is desirable. It has been found that the various cellulose acetates which have been plasticized to improve workability are a particularly suitable oxidizable material. It is desirable to use an oxygenated plasticizer and a polyester of a glycol and a dicarboxylic acid as a particularly suitable plasticizer material. The term oxygenated oxidizable material is intended to include all organic materials which contain oxygen as well as carbon and hydrogen other elements such as nitrogen and sulfur may also be present.

The amount of oxidizable material that is added to the ammonium nitrate-catalyst mixture is dependent upon the amount of catalyst present and the particular type of oxidizable material itself. Normally it is desirable to have present a sufficient amount of oxidizable material so that the explosive mixture is about stoichiometrically balanced with respect to oxygen content. This preferred composition may be exceeded, but in general no more oxidizable material should be present than can react with the available oxygen to yield a soot-free gas. The presence of some oxidizable material is desirable so that the explosive mixture of this invention should contain between about and 25% of oxidizable material, preferably between about 15 and 25% of oxidizable material.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate, such as, conventionally grained ammonium nitrate containing a small amount of impurities and which is then generally coated with a small amount of moisture-resisting material such as petrolatum or paraffin, or military grade ammonium nitrate, or a mixture of other inorganic nitrates and ammonium nitrate wherein the ammonium nitrate is the preponderate nitrate.

The explosive mixture can be made by milling the ingredients or by dry mixing; this operation is preferably followed by forming regular shaped compacted grains by pressing the powdered mix in molds. It is preferred to prepare the shaped grains by adding the powdered ammonium nitrate to fused organic or oxidizable material at l00-125 C., mixing to form a paste, and pressing the paste into suitable molds. The cooled grains are strong and durable.

Test grains of the desired composition were made as follows: When necessary, the ammonium nitrate was ground in a mortar to break up small lumps. The desired amount of ammonium nitrate, oxidizable material and combustion catalyst were weighed into a beaker and the contents thereof were thoroughly mixed. The mixture was extruded by means of a laboratory-size extruder to form a grain about /2 inch in diameter and 6 inches long. In order to insure uniformity, duplicate grains were made and tested in each example. The burning characteristics of each explosive mixture were determined after inhibiting the cylindrical surface of the grain with a thin layer of either asphalt or a cellulose base thermoplastic such. as cellulose methacrylate. Burning rates were determined at atmospheric pressure by igniting the end of the grain and placing a beaker over the ignited grain, thus simulating burning in an inert atmosphere. For tests at elevated pressures, the grain was placed in a Crawford bomb and the bomb brought to the desired operating pressure by means of cylinder nitrogen prior to igniting the grain. Each burning rate represents an average of at least two trials.

Examples In all the examples test grains were made up using fixed amounts of ammonium nitrate, oxidizable material and catalyst. The only difference between the grains lay in the type of combustion catalyst used. All grains were burned in a Crawford bomb at a pressure of 1000 p.s.i.

Each grain contained 73.3 weight percent of military grade ammonium nitrate. The oxidizable material amounted to 24.7 weight percent of the total composition. The binder consisted of one part of Hercules LL-l lacquer grade cellulose acetate; and two parts of dinitro diphenyl oxide; and two parts of the polyester reaction product of ethylene glycol and diglycolic acid in the mol ratio of glycol/ acid of 1.2. In all tests 2 weight percent of combustion catalyst, based on total composition, was

present. In the absence of catalyst this composition would not burn.

Example Catalyst Burning Rate,

inches/second 1 Copper cyanamide 0.12

2 Ammonium Dichromate 0.13

Thus having described the invention, what is claimed is:

1. A composition for the generation of gas which comprises a predominant amount of ammonium nitrate, an oxidizable material and an effective amount of a combustion catalyst selected from at least one metallic cyanamide of the group consisting of barium, copper, lead, mercury and silver, which composition is characterized by a low sensitivity in the absence of said catalyst.

2. An explosive composition which comprises between about 5 and 25 weight percent of an oxidizable material, between about 1 and 25 weight percent of a combustion catalyst selected from at least one metallic cyanamide of the group consisting of barium, copper, lead, mercury and silver, and the remainder essentially ammonium nitrate, which composition is characterized by a low sensitivity in the absence of said catalyst.

3. The composition of claim 2 wherein said cyanamide is copper cyanamide.

4. The composition of claim 2 wherein said cyanamide is lead cyanamide.

5. A composition for the generation of gas at elevated pressures, which composition comprises between about 15 and 25 weight percent of oxygenated oxidizable material, between about 2 and 15 weight percent of copper cyanamide combustion catalyst, and the remainder essentially ammonium nitrate, which composition is characterized by a low sensitivity in the absence of said catalyst.

6. A composition for the generation of gas at elevated pressures, which composition comprises between about 15 and 25 weight percent of oxygenated oxidizable material, between about 2 and 15 weight percent of lead cyan- 2,988,487 7 5 6 V amide combustion catalyst, and the remainder essentially References Cited in the file of this patent ammonium nitrate, which composition is characterized by a low sensitivity in the absence of said catalyst. UNITED STATES PATENTS 7. The composition of claim 2 wherein said oxidizable Cook et a1 Nov. 12, 1940 material is a hydrocarbon. 5 Davis Aug. 22, 1944

Claims (1)

1. A COMPOSITION FOR THE GENERATION OF GAS WHICH COMPRISES A PREDOMINANT AMOUNT OF AMMONIUM NITRATE, AN OXIDIZABLE MATERIAL AND AN EFFECTIVE AMOUNT OF A COMBUSTION CATALYST SELECTED FROM AT LEAST ONE METALLIC CYANAMIDE OF THE GROUP CONSISTING OF BARIUM, COPPER, LEAD, MERCURY AND SILVER, WHICH COMPOSITION IS CHARACTERIZED BY A LOW SENSITIVITY IN THE ABSENCE OF SAID CATALYST.